The invention relates to a method of ensuring the stability of a synchronous machine in controlling the synchronous machine on the basis of direct control of flux and torque, in which method the torque generated by the synchronous machine is increased/decreased by increasing/decreasing the instantaneous angular velocity of the stator flux. The invention thus relates to ensuring and maintaining the stability of a synchronous machine in controlling the synchronous machine on the basis of direct control of flux and torque, known from e.g. U.S. patent publication 4,918,367, which is incorporated herein by reference. The advantages provided by this control method are excellent dynamics and a very accurate follow-up of the reference value for the torque.
To simplify the presentation, the terms stator flux, or simply flux, are used in the specification below and above when referring to stator flux linkage.
FIG. 1 in the attached drawing shows the rotor coordinates of a synchronous machine, i.e. the dq coordinates. The coordinates are fixed to the rotor reference frame, and the positive direction of the d axis (direct axis) is the magnetizing direction of the excitation current (If). In the above control method the torque acting in a positive direction is increased (decreased) by increasing (decreasing) the instantaneous angular velocity of the stator flux (.psi.s), whereby the load angle .delta. increases (decreases). The positive direction of speed and torque is counter clockwise.
The stability of the control of a synchronous machine can be studied by means of FIG. 2 in the attached drawing, showing the static torque generated by a synchronous machine as a function of the load angle as curve T(.delta.). In studying the stability, a static torque curve is used since it is essential that continuous operation is possible at a certain operating point. At torque reference Tref,1, the operation is stable at point .delta.1, since an upward swing of the load angle causes the generated torque to exceed the reference value, resulting in a control action which decreases the load angle. Point .delta.2 is not stable, since a small upward swing of the load angle results in a control action which increases the load angle further. This results in loss of synchronous running. The presented control method does not allow continuous operation if the load angle exceeds the load angle value by which the generated torque reaches its maximum value Tmax. This means that a torque reference Tref,2 exceeding Tmax, does not allow stable operation at all.